A lithium ion secondary battery has established its position as a high-capacity and lightweight power source indispensable for, for example, a portable electronic terminal or an electric car. An active material containing an olivine-type crystal represented by the general formula LiFePO4 has attracted attention as a positive electrode active material for the lithium ion secondary battery. However, lithium poses a concern about a problem such as a worldwide increase in raw material cost. Accordingly, in recent years, a sodium ion secondary battery has been investigated as an alternative to the lithium ion secondary battery. In Patent Literature 1, there is disclosed a positive electrode active material containing a NaxMyP2O7 crystal, where M represents at least one or more kinds of transition metal elements selected from Fe, Cr, Mn, Co, and Ni, x satisfies 1.20≤x≤2.10, and y satisfies 0.95≤y≤1.60.